The magnetoresistance effect that when an external magnetic field is applied to a metal or semiconductor, a change is produced in its resistivity has been utilized in magnetic heads and sensors. To obtain larger magnetoresistance, a magnetoresistance device using a tunnel junction is available, of which the ferromagnetic spin tunnel junction (MTJ) device and the spin injection device have drawn attention. In recent years, the attention has been focused on a magnetoresistance device using the tunnel junction as a new magnetic sensor or a new memory device for a magnetic random access memory (MRAM).
The conventional MTJ device has a ferromagnetic spin tunnel junction made of a layered structure having a ferromagnetic, an insulating and a ferromagnetic layer deposited in this order. When an external magnetic field is applied to control magnetizations of the two ferromagnetic layers to be oriented parallel or antiparallel to each other, the effect that the tunneling current flown in the two directions perpendicular to the film face differ in magnitude from each other, namely the “tunnel magnetoresistance (TMR) effect” as it is called, is obtained at a room temperature (See: T. Miyazaki et al., “Spin polarized tunneling in ferromagnet/insulator/ferromagnet junctions”, 1995, J. Magn. Mater., Springer, Vol. 151, p. 403). The TMR in such a tunnel junction is known to depend on spin polarizability P at an interface between a ferromagnet and an insulator. Assuming the spin polarizabilities of the two ferromagnets to be P1 and P2, then in general TMR can be given by the equation (1) below (See: M. Julliere, “Tunneling between ferromagnetic films”, 1975, Phys. Lett., Vol. 54A, p. 225).TMR=2P1P2/(1−P1P2)  (1)where the spin polarizability P of a ferromagnetic layer has a value in 0<P≦1.
The maximum value of TMR that has currently been obtained at a room temperature in a MTJ device is about 50% of that when a CoFe alloy having a spin polarizability of about 0.5 is used. MTJ devices are expectedly applicable to readout heads for hard disk and MRAMs. When applied to a MRAM, MTJ devices are arrayed in the form of a matrix. Digits 1, 0 are recorded when two magnetic layers constituting a given MTJ device are magnetized either parallel or antiparallel to each other under magnetic fields imposed thereto by flowing electric currents in separately provided another interconnection.
On the other hand, it is known that when the current flows from a ferromagnet to a nonmagnetic metal, if the length of nonmagnetic metal is enough shorter than that of its spin diffusion length, then spins accumulate in the nonmagnetic metal, namely known as “spin accumulation”. Thus, the flowing current from a ferromagnet to a nonmagnetic metal to this effect is called “spin injection”. It was reported that this is due to the fact that in general a ferromagnet has a different spin density (up-spin and down-spin electrons are varied in number) at its Fermi level and this causes the chemical potential difference between up-spin and down-spin electrons when spin polarized electrons are injected by flowing current from the ferromagnet to the nonmagnetic metal (see: M. Johnson et al., “Interfacial Charge-Spin Coupling: Injection and Detection of Spin Magnetism in Metal”, 1985, Phys. Rev. Lett., American Physical Society, Vol. 55, p. 1790).
In a ferromagnet/nonmagnetic metal system in which such spin injection occurs, disposing a second ferromagnet in contact with the nonmagnetic metal if having spins accumulated therein causes a voltage to be induced between the nonmagnetic metal and the second ferromagnet. It was reported that the polarity of the voltage can be inverted either positive or negative when magnetizations of the first and second ferromagnets are controlled to be either parallel or antiparallel to each other (see: M. Johnson et al, “Spin Accumulation”, 1993, Phys. Rev. Lett., American Physical Society, Vol. 70, p. 2142).
A spin injection device has been proposed as a tunnel magnetoresistance device using the spin injection mentioned above (see: F. E. Jedema et al, “Electrical detection of spin precession in a metallic mesoscopic spin valve”, 2002, Nature, Vol. 416, p. 713). FIGS. 5 and 6 illustrate the structure and operating principles of a conventional spin injection device in a cross sectional and a plan view thereof, respectively. As shown in FIG. 5 the conventional spin injection device, designated by reference character 50, comprises a first and a second tunnel junction 51 and 52 for injecting spins and for detecting a voltage due to a spin current, respectively. The first and second tunnel junctions 51 and 52 are spaced apart from each other by a distance L4 that is shorter than a spin diffusion length Ls and is formed on a nonmagnetic metal 53 that serves as a common electrode. The first tunnel junction 51 is constructed of an insulator 54 and a first ferromagnet 55 successively layered on the non-magnetic metal 53 while the second tunnel junction 52 is constructed of an insulator 54 and a second ferromagnet 56 successively layered on the nonmagnetic metal 53. A DC (direct current) source 58 applies a voltage across the first tunnel junction 51. Its positive and negative terminals are connected to the nonmagnetic metal 53 and ferromagnet 55, respectively. The flowing current in the first tunnel junction 51 is designated as I. On the other hand, a voltmeter 59 is connected between the ferromagnet 56 and the nonmagnetic metal 53 in the second tunnel junction 52 for voltage detection.
FIG. 6 is a plan view of essentially what FIG. 5 shows. Here, the spin injection device 50 is shown disposed on a substrate 57. It is also shown that an external magnetic field 60 is applied parallel to the plane of the substrate 57. This applied external magnetic field 60 produces magnetizations 61 and 62 in the ferromagnet 55 of the first tunnel junction 51 and in the ferromagnet 56 of the second tunnel junction 52, respectively. As indicated, patterns shown of the first tunnel junction 51, the second tunnel junction 52 and the nonmagnetic metal 53 have long sides L1, L2 and L3, respectively, and their short sides are W1, W2 and W3, respectively.
Mention is next made of an operation of the conventional spin injection device constructed as described above. In the spin injection device 50, a voltage is applied from the DC source 58 to the first tunnel junction 51 to effect the spin injection by tunneling electrons. A spin current (Is in FIG. 5) by the spin injection flows through a closed circuit to which are connected the voltmeter 59 and the second tunnel junction 52 that is spaced by distance L4 shorter than the spin diffusion length. A voltage induced thereby is detected by the voltmeter 59 connected between the ferromagnet 56 and the nonmagnetic metal 53 in the second tunnel junction 52.
Here, the voltage detection is made easy, since a sign of the induced voltage can be changed by controlling direction of the external magnetic field 60 so that the magnetizations 62 and 63 of the ferromagnets 55 and 56 used in the tunnel junctions 51 and 52 are oriented either parallel or antiparallel to each other. For this reason, the second conventional spin injection device is found promising as a magnetoresistance device using a tunnel junction that is immune to noises.
In the conventional spin injection device, the output resistance Rs for detection can be measured according to the equation below.Rs=(VAP−VP)/Is=Vs/Is  (2)where VAP and VP represent induced voltages when the magnetizations of the ferromagnets 55 and 56 are antiparallel and parallel to each other, respectively. With VS=VAP−VP, Is represents the current flowing through the second tunnel junction 52.
In the conventional spin injection device in which the nonmagnetic metal 53 as the common electrode has small resistance, the problem arises that the detected output resistance is as small as 10 mΩ and fails to provide a practically sufficient signal voltage. Further, in the layered structure of the conventional spin injection device, it is required that the sizes of the first and second tunnel junctions 51 and 52 should be varied each other, since there is the need to invert the magnetization 61 of the ferromagnet 55 in the first tunnel junction 51 at the spin injection side and to fix the magnetization of the ferromagnet 56 in the second tunnel junction 52 at the other. For this reason, the aspect ratio (length/width) of the ferromagnet 56 in the second tunnel junction 52 needs to be larger than that in the first tunnel junction 51.
Also in the conventional spin junction device 50, the distance L4 between the first and second tunnel junctions 51 and 52 must be smaller than the spin diffusion length λN, where λN is generally 1 μm or less. Therefore, the ferromagnet 55, 56 that forms the tunnel junction must be 1 μm or less and further be a submicron or less in size. Since a ferromagnet thus reduced in size increases its demagnetizing field when placed in an external magnetic field, there is increased magnetic switching field that is the magnitude of an external magnetic filed needed for magnetic switching. Thus, the conventional spin injection device has the problem that the magnetic switching field increases when reduced in size. And it can not be operated under the low magnetic field.
Further, in the utilization of conventional spin injection device as memory cells for MRAM, there is a requirement that its device size is made as small as possible to increase their storage capacity. However, the size reduction of the ferromagnet 55, 56 increases magnetic switching field in the conventional spin injection device 50 as mentioned above. This requires that a large current should be flown with a wire separately provided in the MRAM to generate sufficient external field, thus incurring an increase of power consumption. On the other hand, it is necessary to increase the surface area of ferromagnet 51 to some extent in the first tunnel junction 51 for spin injection in order to reduce the power consumption in the field generating wires, permitting flux reversal at low magnetic field in the first tunnel junction 51 for spin injection requires increasing the surface area of its ferromagnet 51 to some extent. Thus, in an attempt to use the conventional spin injection devices 50, e.g., as memory cells in the MRAM, there is the tradeoff relationship between the reducing the magnetic switching field and decreasing the device area in size. Consequently, there has been the problem that limitations are brought about in increasing the storage capacity, e.g., from the fact that reducing magnetic switching field requires increasing the device area in a tunnel junction.